However, the oxidation processes which lead to this sulphone are not always totally satisfactory given that they often lead to an impure product. It is necessary to carry out subsequent purifications, in particular by chromatography, in order to arrive at a product of satisfactory quality.
Moreover, a limited choice of solvents may be used for treating these products. The salts hitherto prepared were soluble in chlorinated solvents or ketones, solvents normally employed for the treatment of pristinamycin II.sub.B sulphones. No acid has allowed until now the preparation of salts which precipitate in the solvents employed.